Laser beam machining method for wiring board, laser beam machining apparatus for wiring board, and carbonic acid gas laser oscillator for machining wiring board

ABSTRACT

In a laser beam machining method for a wiring board, a machined portion of the wiring board is irradiated with a pulsed laser beam for a beam irradiation time ranging from about 10 to about 200 μs and with energy density of about 20 J/cm 2  or more, thereby machining the wiring board, for example, drilling for a through-hole and a blind via hole, grooving, and cutting for an outside shape.

This is a divisional of Application No. 09/549,498 (Confirmation No. NotYet Assigned) filed Apr. 14, 2000 now U.S. Pat. No. 6,373,026, which isa divisional of Application No. 08/690,140, filed Jul. 31, 1996, thedisclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a machining method for a wiring boardand a machining apparatus for a wiring board employing a laser beam formachining such as drilling for a through-hole, an inner via hole, and ablind via hole, grooving, and cutting for an outside shape of the wiringboard referred to as so-called printed board, and more particularly to amachining method for a wiring board and a machining apparatus for awiring board in which a fine conduction hole can rapidly and accuratelybe formed, and a carbonic acid gas laser oscillator to generate a pulsedlaser beam most suitable for the above machining.

2. Description of the Prior Art

A printed board includes a plurality of insulating base materials withconductor layers, stacked and joined in a multi-layer fashion. Among theconductor layers applied onto the insulating base materials, theoptional conductor layers in a vertical direction are electricallyconnected through conduction holes referred to as a through-hole, aninner via hole, and a blind via hole. FIG. 33 is a sectional view ofsuch a conventional multi-layer printed board. In the drawing, referencenumeral 51 means a printed board, 52 to 56 are conductor layers, 57 ismetallic deposits, 61 to 64 are insulating base materials, and 65 to 68are conduction holes. In the five-layer printed board 51 including theconductor layers 52 to 56, the insulating base materials 61 and 63 withboth sides coated with copper foil and the conductor layer 56 includingcopper foil are stacked and joined by using the insulating basematerials 62 and 64 referred to as prepreg, and the conduction holes 65to 68 are provided to pass through the conductor layers 52 to 56.

As shown in FIG. 33, the conduction hole 65 is mounted for conductionbetween the conductor layer 52 and the conductor layer 53 in theinsulating base material 61, and the conduction hole 66, referred to asblind via hole (BVH), is mounted for conduction between the conductorlayer 52 in the insulating base material 61 and the conductor layer 54in the insulating base material 63. The conduction hole 67, referred toas inner via hole (IVH), is mounted for conduction between the conductorlayer 54 and the conductor layer 55 in the insulating base material 63.The conduction hole 68, referred to as through-hole (TH), is mounted forconduction between the conductor layer 52 in the insulating basematerial 61 and the conductor layer 56 stacked and joined through theinsulating base material 64.

The conduction holes 65 to 68 shown in FIG. 33 are holes machined by adrill. Further, after drilling, the conduction holes are plated throughthe metallic deposits 57, and the conductor layers are electricallyconnected.

In the prior art, a machining method for the conduction hole includes,for example, drill machining using a rotary milling cutter. Further, amachining method for grooving or cutting for an outside shape includes,for example, router machining using a rotary milling cutter. On theother hand, in recent years, higher density wiring has been desired forhigher performance of an electronic device. A more multi-layered andsmaller printed board is required to meet the above requirement.Further, it is essential to provide a finer hole diameter of theconduction hole for this purpose. With the current state of the art, theconduction hole is generally provided in the printed board by themechanical method using the drill. However, the method has drawbacks inthat the finer hole diameter is limited, for example, drilling for ahole diameter of φ0.2 mm or less is very difficult to cause heavy wearof the drill such as breakage, resulting in poor productivity due to along time required for replacement of the drill. Further, it isdifficult to simultaneously machine adjacent positions, therebyrequiring a considerable machining time. In addition, the insulatingbase material has a thickness of 0.1 mm or less because of the smallerprinted board. Since it is difficult to control a hole depth in thedrill machining with accuracy of 0.1 mm or less, it is difficult to forma blind via hole in such a thin-walled insulating base material.Further, in order to realize cost reduction by the smaller printed boardand an increase in yield, the grooving and the cutting for the outsideshape require an accurate depth control in the grooving, a narrowercutting width, and cutting after parts are packaged. However, themechanical methods such as router machining are unpractical since theabove limitation is similarly imposed thereon.

Instead of the machining methods for the printed board including theabove mechanical methods, attention has been given to methods, whichhave partially been put to practical use, employing a laser beam such asan eximer laser or carbonic acid gas laser, disclosed in IBM Journal ofResearch and Development, Vol. 126, No.3, pp.306–317 (1982), andJapanese Patent Publication (Kokoku) No. 4-3676. These laser beammachining methods utilize a difference in absorption coefficient oflight energy such as the eximer laser or the carbonic acid gas laserbetween resin or glass fiber serving as the insulating base materialforming the printed board, and copper serving as the conductor layer.For example, since almost the entire laser beam emitted from the abovelaser can be reflected at the copper, a copper foil removed portionhaving a required diameter is formed in top copper foil through etchingand so forth, and the copper foil removed portion may be irradiated withthe laser beam. It is thereby possible to selectively decompose andremove the resin and the glass so as to form a fine through-hole and afine inner via hole in a short time. If internal-layer copper foil ispreviously stacked in a machined portion, the decomposition and theremoval of the insulating base material are terminated at theinternal-layer copper foil. It is thereby possible to form a blind viahole which can surely be terminated at bottom copper foil. There is anadvantage of no wear of the tool such as breakage because the laser beammachining methods are contactless machining methods.

The above laser beam machining methods employ a pulse laser such as theeximer laser and a TEA-carbonic acid gas laser, with an extremely narrowpulse width of 1 μs or less. The pulse laser can finely divide intochips (1) a single base material made of high polymeric material such aspolyimide, or epoxy, (2) a composite material reinforced by aramid fiberor the like, containing the polyimide, the epoxy, and so forth, and (3)an inorganic material such as glass. It is thereby possible to rapidlyand accurately form a good machined hole with a smooth machined portionand less altered layer in a printed board in which as the insulatingbase material is used composite material dispersed in the polyimide, theepoxy, and so forth.

The conventional laser beam machining method for the wiring board hasthe above structure. The eximer laser or the TEA-carbonic acid gas laseris used to provide the through-hole and the inner via hole in the mostcommonly used printed board having the insulating base material made ofglass cloth containing the glass fiber and the resin, such as a glassepoxy printed board referred to as FR-4 made of the glass cloth andepoxy resin. In this case, there are problems in that metallic depositfor conduction can not easily be coated on a hole inner wall due to theextremely rough inner wall of the hole, and reliability of the metallicdeposit can not be ensured. The problems are generated because theinsulating material of the printed board is not only the compositematerial made of organic material and inorganic material but alsoheterogeneous material in which the organic material and the inorganicmaterial are contained in clusters to some extent.

Further, there is another problem in that a uniform machined hole cannot be provided due to differences in, for example, absorptioncoefficient of the laser beam, decomposition temperature, and thermaldiffusivity between an organic material portion and an inorganicmaterial portion. For example, since a wavelength of the laser beam inthe eximer laser can not easily be absorbed by the glass, sufficientenergy for decomposition of the glass can not be supplied so that aglass portion is difficult to remove, resulting in a problem of a roughmachined hole. On the other hand, both the resin and glass can show highabsorption coefficient in case of the TEA-carbonic acid gas laser.However, when energy density of 20 J/cm² required to efficiently machineglass epoxy material is obtained in the TEA-carbonic acid gas laser,excessively high power density of 2×10⁷ W/cm² or more is caused due tothe narrow pulse width of 1 μs or less. Such high power density mayeasily cause plasma at the machined portion. Once the plasma is formed,laser energy is absorbed by the plasma, resulting in insufficient energyreaching the machined portion. Hence, it is difficult to remove glasshaving a high decomposition temperature, thereby causing a problem ofthe rough machined hole.

If the energy density is set to cause no plasma, the machiningprogresses extremely slowly, resulting in a problem of a reduction inproductivity.

In addition, only when a beam diameter is larger than the machinedportion, good machining may be made to the materials (1), (2), and (3)in the conventional laser beam machining method. Otherwise, if themachined portion is larger than the beam diameter, for example, in caseof cutting, grooving, and drilling for a large diameter hole, a removedmaterial caused at a beam irradiated portion adheres to a position otherthan the beam irradiated portion. As a result, after the machining, themachined portion is coated with additionally deposited soot, therebyreducing reliability of insulation and reliability of metallic depositin the printed board. Hence, there is another problem of the need forthe step of, for example, complicated after-treatment such as wetetching.

Other than the extremely short pulse laser such as the eximer laser andthe TEA-carbonic acid gas laser, there are other laser beam machiningmethods for the wiring board, using a typical carbonic acid gas laser ofhigh-speed axial-flow type or three-axes orthogonal type in the priorart. In the conventional carbonic acid gas lasers, more importance isgiven to a CW output characteristic than a pulse output characteristicto enhance oscillation efficiency. That is, there is in theory a problemof pulse response sensitivity at a time of pulse oscillation, inparticular, a characteristic in which a time is required for a fall of alaser pulse. Thus, the machining by the conventional carbonic acid gaslaser having such a characteristic results in irradiation of themachined portion with a laser beam for a longer time, thereby causing agradual temperature gradient around the machined portion. As a result,as shown in FIG. 34, a larger difference is caused in amount of removalbetween the resin and the glass due to a difference in decompositiontemperature therebetween. When only the resin is excessively removed,there are problems in that projection of the glass fibers results in arough machined hole as shown in FIG. 35, and the long heating timegenerates a char layer on a wall surface of the hole.

Further, carbides are generated around the machined portion, and thelaser beam is absorbed by the copper through the carbides, resulting infrequent damage to the copper foil as shown in FIG. 36. Hence, there isa problem in that the blind via hole is difficult to form in the abovelaser beam machining methods.

Though descriptions have been given of the machining for the hole, thesame problems are caused in the grooving and the cutting.

SUMMARY OF THE INVENTION

In order to overcome the above problems, it is an object of the presentinvention to provide a stable laser beam machining method for a wiringboard, in which a printed board with an insulating base materialcontaining cloth-like glass fibers can rapidly and accurately bemachined, for example, drilled for a through-hole, an inner via hole anda blind via hole, grooved, or cut for an outside shape without roughnessof a machined portion and the need for complicated after-treatment ofadditional deposit, and no damage is caused to copper foil. It is alsoan object of the present invention to provide a laser beam machiningapparatus for a wiring board, for realizing the laser beam machiningmethod for the wiring board and improving productivity.

It is another object of the present invention to provide a carbonic acidgas laser oscillator for machining a wiring board, which can output alaser beam with a pulse width most suitable for the laser beam machiningmethod for the wiring board.

According to one aspect of the present invention, for achieving theabove-mentioned objects, there is provided a laser beam machining methodfor a wiring board, including the step of irradiating a machined portionof the wiring board with a pulsed laser beam for a beam irradiation timeranging from 10 to 200 μs and with energy density of 20 J/cm² or more.

According to another aspect of the present invention, there is provideda laser beam machining method for a wiring board, including the step ofirradiating the same machined portion of the wiring board with a pulsedlaser beam with intervals of a beam irradiation pausing time of 15 ms ormore and energy density of 20 J/cm² or more.

According to still another aspect of the present invention, there isprovided a laser beam machining method for a wiring board, including thesteps of combining into one pulse group laser beams including aplurality of pulses having energy density of 20 J/cm² or more andgenerated at intervals of a predetermined beam irradiation pausing time,and irradiating the same machined portion of the wiring board with apulsed laser beam with the plurality of pulse groups respectivelyincluding the plurality of pulses at intervals of a pulse group intervalirradiation pausing time longer than the predetermined beam irradiationpausing time. Preferably, the predetermined beam irradiation pausingtime is 4 ms or more, the number of pulses in the pulse group is 4 orless, and the pulse group interval irradiation pausing time exceeds 20ms.

According to a further aspect of the present invention, there isprovided a laser beam machining method for a wiring board, including thestep of, at a time of scanning a surface of the wiring board whileirradiating a machined portion of the wiring board with the pulsed laserbeam, scanning by a laser beam such that the machined portion is notcontinuously irradiated with the laser beam over 4 pulses at intervalsof a beam irradiation pausing time less than 15 ms.

According to a still further aspect of the present invention, there isprovided a laser beam machining method for a wiring board, including thesteps of providing a 1 mm beam diameter on a surface of a machinedportion, and scanning a surface of the wiring board at a scanning speedranging from 8 to 6 m/min while irradiating the machined portion with alaser beam for a beam irradiation time ranging from 10 to 200 μs and atintervals of a beam irradiation pausing time of 2.5 ms.

According to another aspect of the present invention, there is provideda laser beam machining method for a wiring board, including the steps ofsetting a laser beam to have a square spot effective in machining of amachined portion of the wiring board, and scanning a surface of thewiring board while irradiating the machined portion of the wiring boardwith the pulsed laser beam. Preferably, the square spot of the laserbeam on the machined portion is set to have a size of 0.9 mm×0.9 mm, andthe surface of the wiring board is scanned with a scanning speed of 6m/min and a scanning pitch is 200 μm while the machined portion beingirradiated with the laser beam for a beam irradiation time ranging from10 to 200 μs and at intervals of a beam irradiation pausing time of 1.25ms.

According to a further aspect of the present invention, there isprovided a laser beam machining method for a wiring board, including thesteps of previously removing a metallic layer on the wiring board at aportion corresponding to a machined portion of the wiring board, forminga base material removed portion through machining by irradiating a basematerial of the machined portion with a laser beam through the metalliclayer removed portion, and additionally irradiating the base materialremoved portion and a periphery of the base material removed portion, oronly the periphery of the base material removed portion with a laserbeam. Preferably, the additionally irradiated laser beam has a smallerpeak output than a peak output of the first laser beam, and is used toscan at a higher scanning speed than a scanning speed during first laserbeam irradiation.

According to a further aspect of the present invention, there isprovided a laser beam machining method for a wiring board, including thestep of, at a time of previously removing a metallic layer on the wiringboard at a portion corresponding to a machined portion, partiallyremoving the metallic layer such that a laser beam can reach only anouter periphery of a base material removed portion to be formed byirradiating a base material of the machined portion with the laser beam.Preferably, a surface of the wiring board is scanned with a scanningspeed of 8 m/min and a scanning pitch of 100 μm while the machinedportion being irradiated with the laser beam for a beam irradiation timeranging from 10 to 200 μs and at intervals of a beam irradiation pausingtime of 2.5 ms.

According to a further aspect of the present invention, there isprovided a laser beam machining method for a wiring board, including thesteps of previously removing a metallic layer on the wiring board at aportion corresponding to a machined portion of the wiring board, andflowing a gas in a direction from a laser beam scanning start point to alaser beam scanning end point in the machined portion at a time ofmachining by irradiating a base material of the machined portion with alaser beam while scanning by the laser beam a through the metallic layerremoved portion.

According to a further aspect of the present invention, there isprovided a laser beam machining method for a wiring board, including thesteps of forming a metallic layer having a desired shape by partiallyremoving the metallic layer by pulse irradiation with a laser beamhaving sufficient intensity to melt and remove the metallic layer on thewiring board, and additionally irradiating a machined portion of thewiring board through a metallic layer removed portion with the laserbeam having insufficient intensity to melt the metallic layer and a beamirradiation time ranging from 10 to 200 μs, and including a plurality ofpulses forming a train at intervals of a beam irradiation pausing timeof 15 ms or more. Preferably, the machined portion is exposed bypreviously removing, through another machining method such as etching,the metallic layer positioned at a target position for laser beamirradiation and in the range smaller than a shape to be machined.Further, surface roughening may previously be made to a surface of themetallic layer on a surface of the wiring board before the laser beamirradiation.

According to a further aspect of the present invention, there isprovided a laser beam machining method for a wiring board, including thestep of, at a time of pulse irradiation with a laser beam whilesequentially positioning a spot of the laser beam at target positions onthe wiring board in synchronization with a pulse frequency of the laserbeam, providing a time interval of 15 ms or more between two optionalsuccessive pulsed laser beams for irradiation of the respective targetpositions irrespective of the pulse frequency by irradiating anothertarget position with a pulsed laser beam outputted for the time intervaltherebetween.

According to a further aspect of the present invention, there isprovided a laser beam machining method for a wiring board, including thesteps of providing a plurality of machining stations on which the wiringboards to be machined are mounted, sequentially dividing a pulsed laserbeam outputted from a laser oscillator among the plurality of machiningstations for each pulse, and introducing the pulsed laser beam into theplurality of machining stations at time intervals of 15 ms or more.Preferably, a carbonic acid gas laser is used as a light source of thelaser beam. The wiring board may contain glass cloth.

According to a further aspect of the present invention, there isprovided a laser beam machining apparatus for a wiring board, includingan optical mechanism to change a direction of a laser beam and move thelaser beam on the wiring board while sequentially positioning a spot ofthe laser beam at target positions on the wiring board, and a controlmechanism for synchronous control between a pulse oscillating operationof a laser oscillator and an operation of the optical mechanism, andcontrol of the optical mechanism such that a time interval can be set to15 ms or more between two optional successive pulsed laser beams forirradiation of the target positions irrespective of a pulse frequency ofthe laser oscillator.

According to a further aspect of the present invention, there isprovided a laser beam machining apparatus for a wiring board, includingan optical mechanism to sequentially divide a pulsed laser beamoutputted from a laser oscillator among a plurality of machiningstations for each pulse and introduce the pulsed laser beam into theplurality of machining stations for each pulse at time intervals of 15ms or more, and a synchronization control mechanism for synchronouscontrol between a dividing operation of the optical mechanism and apulse oscillating operation of the laser oscillator. Preferably, theoptical mechanism is provided with at least one rotary chopper rotatedat a predetermined speed of rotation, having a plurality of reflectionsurfaces and a plurality of passing portions at positions equallydividing a periphery about an axis in a plane perpendicular to therotation axis. Further, the synchronization control mechanism isprovided with a trigger generating apparatus to generate a trigger eachtime all the equally divided areas including the plurality of reflectionsurfaces and the plurality of passing portions in the rotary chopperrespectively move across an optical axis of the laser beam.

According to a further aspect of the present invention, there isprovided a carbonic acid gas laser oscillator for machining a wiringboard, in which a length of a discharge space in a gas flow direction isequal to or more than a width of an aperture, an optical axis passingthrough a center of the aperture is set to be positioned in the rangethat an entire area of the aperture does not extend off an areaextending in the gas flow direction of the discharge space and on thefarthest upstream side of the gas flow, and a rise time and a fall timeare set to 50 μs or less in discharge power fed to the discharge space.

The above and further objects and novel features of the invention willmore fully appear from the following detailed description when the sameis read in connection with the accompanying drawings. It is to beexpressly understood, however, that the drawings are for purpose ofillustration only and are not intended as a definition of the limits ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a typical diagram illustrating a laser beam machining methodfor a wiring board according to the embodiment 1 of the presentinvention;

FIG. 2 is a graph diagram showing a relationship between energy densityof a laser beam and a machining depth of a glass epoxy material in thelaser beam machining method for the wiring board according to theembodiment 1 of the present invention;

FIG. 3 is a graph diagram showing a variation in amount of projection ofglass cloth at a machined portion and a variation in rate of damage tocopper foil when a pulse width is varied in the laser beam machiningmethod for the wiring board according to the embodiment 1 of the presentinvention;

FIG. 4 is a typical diagram showing a laser beam machining method for awiring board according to the embodiment 2 of the present invention;

FIG. 5 is a waveform diagram showing an irradiation pattern of a laserbeam in the laser beam machining method for the wiring board accordingto the embodiment 2 of the present invention;

FIG. 6 is a graph diagram showing a variation in thickness of a charlayer observed on the rear side of a machined hole immediately aftermachining when a beam irradiation pausing time is varied in the laserbeam machining method for the wiring board according to the embodiment 2of the present invention;

FIG. 7 is a machined portion temperature characteristic diagram showinga relationship between a distance from a surface of the machined portionand a temperature by using the beam irradiation pausing time as aparameter;

FIG. 8 is a waveform diagram showing an irradiation pattern of a laserbeam in the embodiment 3 of the present invention;

FIG. 9 is a graph diagram showing a variation in thickness of a charlayer when a beam irradiation pausing time is varied between pulsesamong pulse groups in a laser beam machining method for a wiring boardaccording to the embodiment 3 of the present invention;

FIG. 10 is a graph diagram showing a variation in thickness of a charlayer when a pulse group interval beam irradiation pausing time isvaried in the laser beam machining method for the wiring board accordingto the embodiment 3 of the present invention;

FIG. 11 is a graph diagram showing a variation in machining timerequired for drilling when the number of pulses among the pulse groupsis varied in the laser beam machining method for the wiring boardaccording to the embodiment 3 of the present invention;

FIG. 12 is a typical diagram illustrating a laser beam machining methodfor a wiring board according to the embodiment 4 of the presentinvention;

FIG. 13 is an explanatory view showing an existence region of a copperfoil removed portion and a scanning path for raster scanning in thelaser beam machining method for the wiring board according to theembodiment 4 of the present invention;

FIG. 14 is a graph diagram showing a variation in amount of projectionof glass cloth at a machined portion when a scanning speed of a laserbeam is varied in the laser beam machining method for the wiring boardaccording to the embodiment 4 of the present invention;

FIG. 15 is a typical diagram illustrating a laser beam machining methodfor a wiring board according to the embodiment 5 of the presentinvention;

FIG. 16( a) is an explanatory view showing a superimposed portion ofbeam irradiated portions in case of square beam scanning in the laserbeam machining method for the wiring board according to the embodiment 5of the present invention;

FIG. 16( b) is an explanatory view showing a superimposed portion ofbeam irradiated portions in case of circular beam scanning;

FIG. 17 is a typical diagram illustrating a laser beam machining methodfor a wiring board according to the embodiment 6 of the presentinvention;

FIG. 18 is a typical diagram illustrating a laser beam machining methodfor a wiring board according to the embodiment 7 of the presentinvention;

FIG. 19( a) is a plan view showing a machined shape of a copper foilremoved portion in the laser beam machining method for the wiring boardaccording to the embodiment 7 of the present invention;

FIG. 19( b) is a plan view showing a machined shape of a copper foilremoved portion in which copper foil is removed over an entire machinedportion;

FIG. 20 is a typical diagram illustrating a laser beam machining methodfor a wiring board according to the embodiment 8 of the presentinvention;

FIG. 21 is an explanatory view showing a direction of raster scanning bya laser beam, and a gas flow spraying direction in the laser beammachining method for the wiring board according to the embodiment 8 ofthe present invention;

FIG. 22 is a typical diagram illustrating a laser beam machining methodfor a wiring board according to the embodiment 9 of the presentinvention;

FIG. 23 is a typical diagram showing the result of machining of aprinted board according to the embodiment 9 of the present invention;

FIG. 24 is a typical diagram showing a laser beam machining method for awiring board according to the embodiment 10 of the present invention;

FIG. 25 is a typical diagram showing a laser beam machining method for awiring board according to one modification of the embodiment 10 of thepresent invention;

FIG. 26 is a typical diagram showing a laser beam machining method for awiring board, and a laser beam machining apparatus for a wiring boardaccording to the embodiment 11 of the present invention;

FIG. 27 is a typical diagram showing a laser beam machining method for awiring board, and a laser beam machining apparatus for a wiring boardaccording to the embodiment 12 of the present invention;

FIG. 28 is a typical diagram of a rotary chopper according to theembodiment 12 of the present invention;

FIG. 29 is a time chart of a trigger and laser pulses in the embodiment12 of the present invention;

FIG. 30 is a perspective view of a carbonic acid gas laser oscillatorfor machining a wiring board according to the embodiment 13 of thepresent invention;

FIGS. 31( a) and 31(b) are typical diagrams respectively showing a gaindistribution and arrangement of an optical axis in a discharge space ina conventional carbonic acid gas laser oscillator;

FIG. 32 is a typical diagram showing arrangement of an optical axis inthe carbonic acid gas laser oscillator for machining the wiring boardaccording to the embodiment 13 of the present invention;

FIG. 33 is a sectional view showing a structure of a conventionalmulti-layer printed board;

FIG. 34 is a graph diagram showing a mechanism to generate a reductionin quality in a conventional laser beam machining method for a wiringboard;

FIG. 35 is a sectional view of a machined portion, showing projection ofglass cloth and a thickness of a char layer in the conventional laserbeam machining method for the wiring board; and

FIG. 36 is a sectional view of a machined portion, showing damage tocopper foil in the conventional laser beam machining method for thewiring board.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will now be given of preferred embodiments of the presentinvention.

Embodiment 1

FIG. 1 is a typical diagram showing a laser beam machining method for awiring board according to the embodiment 1 of the present invention. Inthe drawing, reference numeral 1A means a printed board (wiring board),2, 3, and 4 are conductor layers (metallic layers) including copperfoil, 8 is a copper foil removed portion formed in the top conductorlayer 2 by etching, 9 is a ZnSe lens for convergence of a laser beam 27radiated from a carbonic acid gas laser, and 10 is an assist gas forlens protection. Air is employed in the embodiment as the assist gas 10.Reference numerals 11 and 12 are insulating base materials, and 19 is agas nozzle through which the assist gas 10 is ejected. Here, the copperfoil removed portion 8 is formed in the conductor layer 2 at a portioncorresponding to a machined portion of the insulating base material 11.

In the embodiment 1, a three-layer glass epoxy printed board (FR-4) withboth sides coated with copper foil and a thickness of 200 μm is used asthe multi-layer printed board 1A. Further, the copper foil has athickness of 18 μm in the conductor layers 2, 3, and 4, and the copperfoil removed portion 8 with a diameter of 200 μm is formed in the topconductor layer 2 by the etching.

A description will now be given of the operation.

FIG. 2 is a graph diagram showing the result of machining in which thecarbonic acid gas laser is used as a light source, and energy per pulseis varied in the laser beam 27 therefrom, thereby varying energy densityin the range from 7 to 35 J/cm² at the copper foil removed portion 8corresponding to the machined portion of the printed board 1A, andirradiating an exposed portion of the insulating base material 11 withonly one pulse through the copper foil removed portion 8. In the graph,the transverse axis defines energy density (J/cm²), and the ordinateaxis defines a machining depth (μm) of a glass epoxy material. As isapparent from FIG. 2, a variation in energy density per pulse of thelaser beam 27 varies the machining depth of the printed board 1A made ofthe glass epoxy material. If the energy density is 20 J/cm² or less,machining is carried out with a negligible amount of removal. Thus, itis necessary to irradiate with a lot of pulses so as to pass through theglass epoxy material with a thickness of 100 μm. In view ofproductivity, a through-hole should be formed by several pulses perhole. Therefore, from the result of experiment shown in FIG. 2, it canbe seen that high-speed and efficient machining requires irradiationwith the laser beam 27 with the energy density of 20 J/cm² or more.

FIG. 3 is a graph diagram showing the result of machining in which thelaser beam 27 per pulse has constant energy density of 200 mJ, the laserbeam 27 is condensed through the ZnSe lens 9 so as to provide a beamdiameter of 500 μm on a surface of the machined portion of the printedboard 1A, and set energy density to 100 J/cm², and the copper foilremoved portion 8 is irradiated with only one pulse with a pulse widthvarying in the range of 1 to 500 μs. In the drawing, the transverse axisdefines the pulse width (μs), and the ordinate axes define an amount ofprojection (μm) of glass cloth and a ratio (%) of damage to the copperfoil. In this case, the air was used as the assist gas 10 for lensprotection, and was supplied to the machined portion through the gasnozzle 19 at a flow rate of 10 1/min.

It is possible to check the amount of projection of the glass cloth in amachined hole (or the base material removed portion) at a time ofvarying the pulse width of the laser beam 27, by observing a section ofthe machined hole through a microscope as shown in FIG. 35. In FIG. 3,variations in maximum value of the amount of projection of the glasscloth and in ratio of damage to the copper foil are plotted versus avariation in pulse width ranging from 1 to 500 μs. The ratio of damageto the copper foil is expressed as a percentage by using the number ofmachined holes passing through the conductor layer 3 serving as thebottom copper foil among 1,000 machined holes. As shown in FIG. 3, whenthe pulse width of the laser beam 27 is set in the range from 10 to 200μs, it is possible to provide a machined hole with a little amount ofprojection of the glass cloth, and no damage to the bottom copper foil.In such a manner, by setting a beam irradiation time to 200 μs or less,it is possible to provide a sharp temperature gradient of a machinedportion during machining (hereinafter, the machined portion as usedherein meaning, for example, the machined hole during or after themachining) in the printed board 1A in the range from a surface of themachined portion to an inside portion. Further, it is possible to reducethe amount of projection of the glass cloth to a substantiallynegligible extent. In addition, since an amount of generating carbidesis reduced, the damage to the copper foil can be reduced, and a blindvia hole can stably be formed.

After ultrasonic cleaning and desmearing of the obtained machined hole,the machined hole was plated with copper, and a pattern was formed,thereafter observing the section of the hole. It came clear that plasmawas caused at the machined portion during the laser beam machining whenthe laser beam 27 had a pulse width less than 10 μs, and the glass clothwas not completely removed due to the plasma. As a result, the bottomcopper foil was not completely conductive in spite of the plating sothat the many machined holes could not serve as a through-hole. On theother hand, when the laser beam 27 had a pulse width in the range from10 to 200 μs, it was possible to provide a good through-hole in whichall copper foil including the bottom copper foil were completelyconductive through the plating. For comparison, though the samemachining was carried out by using a diamond drill with a diameter of200 μm, it was difficult to control a depth. That is, 10% of the 1,000machined holes passed through the conductor layer 4 serving as bottomcopper foil so that the conductor layer 3 and the conductor layer 4 wereshort-circuited. As described above, through the drill machining, it wasdifficult to provide an effect identical with that achieved by the laserbeam machining method for the wiring board according to the embodiment1.

As set forth above, according to the embodiment 1, at a time ofirradiating the machined portion with the laser beam 27 having energydensity of 20 J/cm² or more required for efficient machining of theprinted board 1A made of the glass epoxy material containing the glasscloth and the epoxy resin, the beam irradiation time is appropriatelyset in the range from 10 to 200 μs. Since it is thereby possible toreduce power density to 2×10⁶ W/cm² or less, the machining can becarried out without the plasma generating at the machined portion.Further, by setting the beam irradiation time to 200 μs or less, it ispossible to provide the sharp temperature gradient of the machinedportion during machining in the printed board 1A in the range from thesurface of the machined portion to the inside portion. Further, it ispossible to reduce the amount of projection of the glass cloth to thesubstantially negligible extent. In addition, since the amount ofgenerating carbides can be reduced, the damage to the copper foil can bereduced, and the blind via hole can stably be formed.

Embodiment 2

FIG. 4 is a typical diagram showing a laser beam machining method for awiring board according to the embodiment 2 of the present invention. Inthe drawing, the same reference numerals are used for component partsidentical with those in FIG. 1, and descriptions thereof are omitted.Further, in FIG. 4, reference numeral 1B means a multi-layer printedboard, 5 is a conductor layer, 6 is a conductor layer on a rear surfaceof the multi-layer printed board 1B, 7 is metal deposited on an innersurface of a through-hole 17, and 13 and 14 are insulating basematerials. FIG. 5 is a waveform diagram showing an irradiation patternof a laser beam 27 in the embodiment 2.

In the embodiment 2, a five-layer glass polyimide board with a thicknessof 400 μm was used as the printed board 1B. In a top conductor layer 2and a bottom conductor layer 6, copper foil had a thickness of 18 μm,and etching was made to form copper foil removed portions 8 with adiameter of 200 μm in the conductor layer 2 and the conductor layer 6 atpositions corresponding to a conduction hole to be machined.

A description will now be given of the operation.

A carbonic acid gas laser with a pulse width of 50 μs and pulse energyof 280 mJ was condensed on the printed board 1B through a ZnSe lens 9such that a laser beam diameter became 500 μm on a surface of themachined portion, thereby setting energy density to 143 J/cm². Further,a beam irradiation pausing time shown in FIG. 5 was varied in the rangefrom 12.5 to 50 ms, and a portion of the insulating base material 11exposed through the copper foil removed portion 8 was irradiated withthe pulsed laser beam 27. At the time, air was used as an assist gas 10for lens protection, and was supplied to the machined portion through agas nozzle 19 at a flow rate of 10 1/min. FIG. 6 is a graph diagramshowing a variation in thickness (μm) of a char layer observed on therear side of the machined hole immediately after the machining when thebeam irradiation pausing time is varied as described above. It ispossible to check the thickness of the char layer by observing a sectionof the machined hole through a microscope as shown in FIG. 35.

As shown in FIG. 6, when the beam irradiation pausing time is below 15ms, the thickness of the char layer rapidly increases. After the laserbeam machining, the obtained printed board 1B was ultrasonically cleanedby pure water for three minutes. It was thereby possible to completelyremove the char layer in case of the beam irradiation pausing time of 15ms or more. After the ultrasonic cleaning and desmearing of the obtainedmachined hole, the machined hole was plated with copper, and a patternwas formed, thereafter observing the section. It was possible to providea good through-hole having a diameter of 200 μm and a smooth inner wallin case of the beam irradiation pausing time of 15 ms or more. On theother hand, in case of the beam irradiation pausing time less than 15ms, a residual char layer and projecting glass cloth were observedbetween a metallic deposit and a base material of the printed board 1B,the hole had a rough inner wall, and throwing power in plating wasinsufficient.

It can be considered that, as shown in FIG. 7, the above problems werecaused because, in case of the beam irradiation pausing time less than15 ms, a gradual temperature gradient was provided by machiningaccording to a distance from a surface of the machined portion duringthe machining, and a temperature was excessively increased at a deepportion from the surface of the machined portion, at which a rise of thetemperature was unnecessary. On the other hand, if the same beamirradiated portion is irradiated with the pulsed laser beam 27 with thebeam irradiation pausing time of 15 ms or more, it is possible to ensurea cooling time required to completely cool the machined portion for eachpulse. As shown in FIG. 7, in case of the beam irradiation pausing timeof 15 ms or more, it is possible to reduce gradation of the temperaturegradient caused due to the temperature rise at the machined portionduring irradiation with the laser beam 27, and reduce the projection ofthe glass cloth.

As set forth above, the carbonic acid gas laser is used for multi-pulseirradiation at appropriate irradiation intervals. It is thereby possibleto provide the conduction hole having a high aspect ratio which can notbe obtained by a single pulse, and rapidly machine the printed boardincluding the glass cloth with high accuracy.

For comparison, though the same machining was carried out by using adiamond drill with a diameter of 200 μm, the drill was worn out aftermachining for about 1,000 machined holes, resulting in the rough innerwall of the hole and occasional breakage of the drill. Hence, the methodrequired a machining time about ten times a machining time in the laserbeam machining method for the wiring board according to the embodiment2.

As set forth above, according to the embodiment 2, since the same beamirradiated portion is irradiated with the pulsed laser beam at intervalsof the beam irradiation pausing time of 15 ms or more, it is possible toensure the cooling time required to completely cool the machined portionfor each pulse. As shown in FIG. 7, it is possible to increase thetemperature gradient of the machined portion, and reduce heating at themachined portion. As a result, it is possible to reduce the projectionof the glass cloth, and rapidly machine the printed board containing theglass cloth with high accuracy even in case of multi-pulse irradiation.

Embodiment 3

FIG. 8 is a waveform diagram showing an irradiation pattern of a laserbeam in a laser beam machining method for a wiring board according tothe embodiment 3 of the present invention. In the embodiment 3, as shownin FIG. 4 of the embodiment 2, a five-layer glass polyimide board with athickness of 400 μm was used as a printed board 1B. In a top conductorlayer 2 and a bottom conductor layer 6, copper foil had a thickness of18 μm, and etching was made to form copper foil removed portions 8 witha diameter of 200 μm in the conductor layer 2 and the conductor layer 6at positions corresponding to a conduction hole to be machined.

A description will now be given of the operation.

A laser beam 27 was emitted from a carbonic acid gas laser with aconstant pulse width of 50 μs, and constant pulse energy of 280 mJ.Further, the laser beam 27 was condensed on the printed board 1B througha ZnSe lens 9 such that a laser beam diameter became 500 μm on a surfaceof the machined portion, thereby setting energy density to 143 J/cm². Asshown in FIG. 8, there were provided a plurality of pulse groupsrespectively including two to ten pulses with a beam irradiation pausingtime of t1, and the printed board 1B was irradiated with the pulsegroups for a pulse group interval beam irradiation pausing time of t2.

In the embodiment, the beam irradiation pausing time t1 was varied inthe range from 0 to 10 ms, and the pulse group interval beam irradiationpausing time t2 was varied in the range from 50 to 10 ms. A portion ofan insulating base material 11 exposed through the copper foil removedportion 8 was irradiated with 52 pulses. At the time, air was used as anassist gas 10 for lens protection, and was supplied to the machinedportion through a gas nozzle 19 at a flow rate of 10 1/min.

FIG. 9 is a graph diagram showing a variation in thickness of a charlayer observed on the rear side of a machined hole immediately after themachining when the beam irradiation pausing time t1 was varied betweenpulses among the pulse groups. In this case, the pulse group intervalbeam irradiation pausing time t2 was set to a sufficiently large valueof 50 ms. As shown in FIG. 9, when the beam irradiation pausing time t1was equal to or more than 4 ms, the thickness of the char layer becamethinner than a thickness (ranging from 50 to about 100 μm) of the layerin case of the beam irradiation pausing time t1 of 0 ms. Thus, it can beseen that the variation in beam irradiation pausing time t1 caneffectively reduce the thickness of the char layer.

FIG. 10 is a graph diagram showing a variation in thickness of a charlayer observed on the rear side of a machined hole immediately after themachining when the pulse group interval beam irradiation pausing time t2was varied from 50 to 10 ms. In this case, the number of pulses amongthe pulse groups was set to two, and the beam irradiation pausing timet1 was set to 10 ms. As shown in FIG. 10, when the pulse group intervalbeam irradiation pausing time t2 was 20 ms or less, the thickness of thechar layer rapidly increased.

FIG. 11 is a graph diagram showing a variation in thickness of a charlayer observed on the rear side of a machined hole immediately after themachining with respect to a variation in machining time required fordrilling when the number of pulses among the pulse groups was varied. Atthe time, the beam irradiation pausing interval t1 between the pulseswas set to 25 ms, and the pulse group interval beam irradiation pausingtime t2 was set to 50 ms. As shown in FIG. 11, when the number of pulsesis 4 or less, with the same machining quality, it is possible to reduceabout 6 to 22% of a machining time required for machining by a singlepulse frequency.

After ultrasonic cleaning and desmearing of the obtained machined hole,the machined hole was plated with copper, and a pattern was formed,thereafter observing a section. When the beam irradiation pausing timet1 between pulses was 4 ms or more, the pulse group interval beamirradiation pausing time t2 was 20 ms or more, and the number of pulsesin the pulse groups was 4 or less, it was possible to provide a goodthrough-hole having a diameter of 200 μm and a smooth inner wall as inthe case of the single pulse frequency. Further, in case of athin-walled printed board, it was possible to provide a goodthrough-hole even when the number of pulses among the pulse groups was 4or more with satisfaction of the above conditions of the beamirradiation pausing time t1 and the pulse group interval beamirradiation pausing time t2. That is, it was possible to reduce themachining time by satisfying the conditions of the beam irradiationpausing time t1 and the pulse group interval beam irradiation pausingtime t2 so as to appropriately select the number of pulses among thepulse groups according to a thickness of the printed board. Further, ifthe conditions of the beam irradiation pausing time t1 and the pulsegroup interval beam irradiation pausing time t2 were not met, a residualchar layer and projecting glass cloth were observed between a metallicdeposit and a base material of the printed board 1B, the hole had arough inner wall, and throwing power in plating was insufficient.

As set forth above, according to the embodiment 3, the appropriate beamirradiation pausing time is provided, and the multi-pulse irradiation iscarried out with the pulse groups including the several pulses. It isthereby possible to reduce the machining time to be less than that incase of the single pulse. At the same beam irradiated portion, themachined portion is irradiated with the pulsed laser beam having theplurality of pulse groups respectively including the plurality of pulseswith intervals of the predetermined beam irradiation pausing time withintervals of the pulse group interval beam irradiation pausing timelonger than the beam irradiation pausing time between the pulses. It isthereby possible to prevent a rise of a temperature at the machinedportion, reduce gradation of the temperature gradient with respect to adepth distance from the surface of the machined portion, and reduce theprojection of the glass cloth.

Embodiment 4

FIG. 12 is a typical diagram illustrating a laser beam machining methodfor a wiring board according to the embodiment 4 of the presentinvention. In the drawing, the same reference numerals are used forcomponent parts identical with or equivalent to those in FIG. 1, anddescriptions thereof are omitted. In the embodiment, a three-layer glassepoxy printed board (FR-4) with a thickness of 500 μm was used as amulti-layer printed board 1C. Further, copper foil had a thickness of 18μm in conductor layers 2, 3, and 4, a distance between the conductorlayer 2 and the conductor layer 3 was 200 μm, and a copper foil removedportion 8 with a diameter of 200 μm was formed in the top conductorlayer 2 by etching.

A description will now be given of the operation.

A laser beam 27 was emitted from a carbonic acid gas laser with constantpulse energy of 280 mJ, a constant pulse width of 50 μs and a constantpulse frequency of 400 Hz. Further, the laser beam 27 was condensed onthe printed board 1C through a ZnSe lens 9 such that a laser beamdiameter became 1 mm on a surface of a machined portion, thereby settingenergy density to 35 J/cm². As shown in FIG. 13, a raster scanning wasmade on a path 26 with a scanning speed ranging from 8 to 3 m/min and ascanning pitch of 100 μm such that an entire existence region 25 of thecopper foil removed portion 8 was irradiated with the laser beam 27. Atthe time, air was used as an assist gas 10 for lens protection, and wassupplied to the machined portion through a gas nozzle 19 at a flow rateof 10 l/min.

FIG. 14 is a graph diagram showing a variation in amount of projectionof glass cloth in a machined hole when the scanning speed of the laserbeam 27 is varied. In the drawing, as the amount of projection of theglass cloth, the maximum value thereof is plotted. As shown in FIG. 14,when the scanning speed of the laser beam 27 is in the range of 8 to 6m/min, it is possible to provide a machined hole with a small amount ofprojection of the glass cloth and no damage to bottom copper foil.

After ultrasonic cleaning and desmearing of the obtained machined hole,the machined hole was plated with copper, and a pattern was formed,thereafter observing a section. When the scanning speed of the laserbeam 27 was 6 m/min or less, the amount of projection of the glass clothbecame 20 μm or more due to heat, resulting in insufficient throwingpower. As a result, there were observed many printed boards havingplating solution soaking into the glass cloth. On the other hand, whenthe scanning speed of the laser beam 27 was in the range from 8 to 6m/min, it was possible to highly efficiently provide a good conductionhole in which all the copper foil including the bottom copper foil werecompletely conductive through the plating.

As set forth above, according to the embodiment 4, it is possible totremendously increase a machining speed while keeping the same machiningquality as that of the same machining by positioning the laser beam 27for each machined portion. Further, it is possible to reduce, forexample, the projection of the glass cloth during the machining of theprinted board, thereby enabling high quality machining such as drillingof a through-hole and a blind via hole, grooving, and cutting for anoutside shape.

Embodiment 5

FIG. 15 is a typical diagram illustrating a laser beam machining methodfor a wiring board according to the embodiment 5 of the presentinvention. In the drawing, the same reference numerals are used forcomponent parts identical with those in FIG. 1, and descriptions thereofare omitted. Reference numeral 48 means a beam shaping optical system toshape a laser beam 27 through a kaleidoscope such that the laser beam 27can have a beam spot size of 0.9 mm×0.9 mm on a surface of a machinedportion.

In the embodiment 5, as in the embodiment 4, a three-layer glass epoxyprinted board (FR-4) with a thickness of 500 μm was used as a printedboard 1C. Further, copper foil had a thickness of 18 μm in conductorlayers 2, 3, and 4, a distance between the conductor layer 2 and theconductor layer 3 was 200 μm, and a copper foil removed portion 8 with adiameter of 200 μm was formed in the top conductor layer 2 by etching.

A description will now be given of the operation.

A laser beam 27 was emitted from a carbonic acid gas laser with constantpulse energy of 280 mJ, a constant pulse width of 50 μs and a constantpulse frequency of 800 Hz. Further, the laser beam 27 was condensed onthe multi-layer printed board 1C through a ZnSe lens 9 after shaping byusing the beam shaping optical system 48 including the kaleidoscope suchthat the laser beam 27 had the beam spot size of 0.9 mm×0.9 mm on thesurface of the machined portion, thereby setting energy density to 35J/cm². As in the embodiment 4, a raster scanning was made with ascanning speed of 6 m/min and a scanning pitch of 200 μm such that anentire existence region of the copper foil removed portion 8 wasirradiated with the laser beam 27. At the time, air was used as anassist gas 10 for lens protection, and was supplied to the machinedportion through a gas nozzle 19 at a flow rate of 10 l/min. Further, forcomparison, the same laser beam machining was made by a circular beamwith the same energy density and a diameter of 1 mm.

As a result, as shown in FIG. 16( a), when a path 26 is scanned by asquare laser beam 27 a having a square laser beam on a machined portion21, it was possible to provide a machined hole with a small amount ofprojection of glass cloth and no damage to bottom copper foil. On theother hand, as shown in FIG. 16( b), in case of a circular laser beam 27b, there were caused carbonization in the machined hole and breakage ofthe bottom copper foil.

This is because, as shown in FIGS. 16( a) and 16(b), a superimposedportion 24 of beam irradiated portions can more be reduced in scanningby the square laser beam 27 a having the square laser beam on themachined portion 21 of the printed board 1C than would be in scanning bythe circular laser beam 27 b. As a result, the square laser beam 27 acan decrease gradation of a temperature gradient generated according toa temperature rise at the machined portion, and can more reduce a lowerlimit of a beam irradiation pausing time than would be in the circularlaser beam 27 b. By scanning the surface of the printed board 1C by thepulse carbonic acid gas laser, it is possible to carry out the machiningof the printed board 1C such as drilling for a through-hole and a blindvia hole, grooving, and cutting for an outside shape at a more rapidmachining speed than that in case of the circular laser beam 27 b,resulting in the same machining quality.

After ultrasonic cleaning and desmearing of the obtained machined hole,the machined hole was plated with copper, and a pattern was formed,thereafter observing a section. In case of the circular laser beam 27 b,the amount of projection of the glass cloth became 20 μm or more due toheat, resulting in insufficient throwing power. As a result, there wereobserved many printed boards having plating solution soaking into theglass cloth. On the other hand, in case of the square laser beam 27 a,it was possible to provide a good conduction hole in which all thecopper foil including the bottom copper foil were completely conductivethrough the plating.

As set forth above, according to the embodiment 5, the square laser beamis provided on the surface of the sample. It is thereby possible toprovide a more rapid machining speed than that in case of the circularlaser beam 27 b while keeping good machining quality.

Embodiment 6

FIG. 17 is a typical diagram illustrating a laser beam machining methodfor a wiring board according to the embodiment 6 of the presentinvention. In the drawing, the same reference numerals are used forcomponent parts identical with those in FIG. 1, and descriptions thereofare omitted. Reference numeral 1D means a printed board, and a glassepoxy printed board (FR-4) with a thickness of 200 μm and both sidescoated with copper is used as the printed board 1D. Copper foil has athickness of 18 μm in conductor layers 2 and 3. Etching with a pitch of10 mm is made to remove copper foil with a width of 1 mm and a length of10 mm so as to form copper foil removed portions 8 in the top conductorlayer 2 and the bottom conductor layer 3 in the printed board 1D at thesame position.

A description will now be given of the operation.

According to the embodiment 6, a laser beam 27 was emitted from acarbonic acid gas laser with constant pulse energy of 280 mJ, a constantpulse width of 50 μs and a constant pulse frequency of 400 Hz. Further,the laser beam 27 was condensed on the printed board 1D through a ZnSelens 9 such that the laser beam 27 had a beam diameter of 1 mm on asurface of a machined portion, thereby setting energy density to 35J/cm². As shown in FIG. 13, a raster scanning was made with a scanningpitch of 100 μm and a scanning speed of 8 m/min such that an entireexistence region 25 of the copper foil removed portion 8 was irradiatedwith the laser beam 27. At the time, air was used as an assist gas 10for protection of the ZnSe lens 9, and was supplied to the machinedportion through a gas nozzle 19 at a flow rate of 10 l/min. Though noglass cloth projected and no char layer was generated, there were rigidresidual additionally deposited around the machined hole due to largevolumes of removed base materials.

After the machining, the laser beam 27 was emitted from the carbonicacid gas laser with the constant pulse energy of 200 mJ, the constantpulse width of 50 μs and the constant pulse frequency of 400 Hz.Further, the laser beam 27 was condensed on the printed board 1D throughthe ZnSe lens 9 such that the laser beam 27 had the beam diameter of 1mm on the surface of the machined portion, thereby setting the energydensity to 25 J/cm². As in the above machining, a second raster scanningwas made with a scanning speed of 10 m/min and a scanning pitch of 100μm such that an entire existence region 25 of the copper foil removedportions 8 was irradiated with the laser beam 27. At the time, air wasused as the assist gas 10 for protection of the ZnSe lens 9, and wassupplied to the machined portion through the gas nozzle 19 at the flowrate of 10 l/min. This can substantially completely remove theadditional deposits around the machined hole without damage to the topcopper foil.

After ultrasonic cleaning and desmearing of the obtained machined hole,the machined hole was plated with copper, and a pattern was formed,thereafter observing a section. It was possible to provide a good slitwhich was completely conductive through the plating without the residualadditional deposit around the machined hole.

As set forth above, according to the embodiment 6, after the basematerial is removed by beam irradiation, the machined hole and aperiphery of the machined hole, or only the periphery of the machinedhole is irradiated with the laser beam 27 once again to remove sootadditionally deposited around the machined hole. The second beamirradiation is used to remove only the soot, resulting in a small amountof removal and no additionally deposited soot. Thereby, even when aportion to be machined is larger than the laser beam diameter, forexample, even in case of cutting, grooving, or drilling for a largediameter hole, it is possible to remove the additional deposits withoutthe step for complicated after-treatment such as wet etching to removethe additionally deposited soot serving as the residual in the machinedhole after the machining. As a result, it is possible to avoid areduction in reliability of insulation and reliability of metallicdeposit in the printed board.

Embodiment 7

FIG. 18 is a typical diagram illustrating a laser beam machining methodfor a wiring board according to the embodiment 7 of the presentinvention. In the drawing, the same reference numerals are used forcomponent parts identical with those in FIG. 1, and descriptions thereofare omitted. Reference numeral 18 means copper foil removed portions. Inthe embodiment 7, as in the embodiment 6, as a printed board 1D was useda glass epoxy printed board (FR-4) with a thickness of 200 μm and bothsides coated with copper. Further, copper foil had a thickness of 18 μmin conductor layers 2 and 3. Etching with a pitch of 2 mm was made toform copper foil removed portions 8 with a width of 1 mm and a length of10 mm in the top conductor layer 2 and the bottom conductor layer 3 onthe printed board 1D at the same position. As shown in FIG. 19( a), thecopper foil removed portion 18 was formed by removing copper foil with awidth of 100 μm corresponding to only an outer periphery 18 a of thecopper foil removed portion 18 through the etching. Further, for thepurpose of confirmation of an effect in case the copper foil removedportion 18 was used, another etching was made to entirely remove aportion corresponding to a machined portion as in the embodiment 6 so asto form a copper foil removed portion 8 as shown in FIG. 19( b).

A description will now be given of the operation.

A laser beam 27 was emitted from a carbonic acid gas laser with constantpulse energy of 280 mJ, a constant pulse width of 50 μs and a constantpulse frequency of 400 Hz. Further, the laser beam 27 was condensed onthe printed board 1D through a ZnSe lens 9 such that the laser beam 27had a beam diameter of 1 mm on a surface of a machined portion, therebysetting energy density to 35 J/cm². As in the embodiment 6, a rasterscanning was made with a scanning speed of 8 m/min and a scanning pitchof 100 μm such that an entire existence region of the copper foilremoved portion 18 was irradiated with the laser beam 27. At the time,air was used as an assist gas 10 for lens protection, and was suppliedto the machined portion through a gas nozzle 19 at a flow rate of 10l/min.

As a result, in the printed board in which only the outer periphery 18 aof the copper foil removed portion 18 was machined, it was possible toform a good slit without projection of glass cloth, generation of a charlayer, and rigid additional deposits on a periphery of a machined hole.On the other hand, in the printed board with the copper foil removedportion 8 formed by entirely removing the portion corresponding to themachined portion as shown in FIG. 19( b), as described above, no glasscloth projected and no char layer was generated. However, there wererigid residual additionally deposited around the machined hole due tolarge volumes of removed base materials.

After ultrasonic cleaning and desmearing of the printed board 1Dobtained by machining only the outer periphery 18 a of the copper foilremoved portion 18, the printed board was plated with copper, and apattern was formed thereon, thereafter observing a section. It waspossible to provide a good slit which was completely conductive throughthe plating without the residual additional deposit around the machinedhole and peeling of the copper foil.

As set forth above, according to the embodiment 7, since only the outerperiphery 18 a of the copper foil removed portion 18 is machined, avolume of removal can be reduced at a time of machining so that themachined hole having the same shape can be provided after the machining.At the time, since a small volume of machined materials can reduce atemperature rise around the machined hole, it is possible to reducegradation of a temperature gradient as shown in FIG. 7. That is, alarger temperature gradient can be provided to enable good machiningcausing no failure such as peeling of the copper foil even in case ofmachining with the removed portion larger than a non-removed portion.Further, a shorter beam irradiation pausing time can be provided thanthat in a method of irradiating the entire machined portion with thebeam, resulting in higher speed machining.

Embodiment 8

FIG. 20 is a typical diagram illustrating a laser beam machining methodfor a wiring board according to the embodiment 8 of the presentinvention. In the drawing, the same reference numerals are used forcomponent parts identical with those in FIG. 1, and descriptions thereofare omitted. In the embodiment 8, as in the embodiment 6, as a printedboard 1D serving as a machining target was used a glass epoxy printedboard (FR-4) with a thickness of 200 μm and both sides coated withcopper. Further, copper foil had a thickness of 18 μm in conductorlayers 2 and 3. Etching with a pitch of 10 mm was made to form copperfoil removed portions 8 with a width of 1 mm and a length of 10 mm inthe top conductor layer 2 and the bottom conductor layer 3 on theprinted board 1D at the same position.

A description will now be given of the operation.

A laser beam 27 was emitted from a carbonic acid gas laser with constantpulse energy of 280 mJ, a constant pulse width of 50 μs and a constantpulse frequency of 400 Hz. Further, the laser beam 27 was condensed onthe printed board 1D through a ZnSe lens 9 such that the laser beam 27had a beam diameter of 1 mm on a surface of a machined portion, therebysetting energy density to 35 J/cm². As shown in FIG. 21, a rasterscanning was made along a path 26 with a scanning speed of 8 m/min and ascanning pitch of 100 μm such that an entire existence region 25 of thecopper foil removed portion 8 was irradiated with the laser beam 27. Atthe time, air was used as an assist gas 10, and was sprayed and suppliedto the machined portion at a flow rate of 50 l/min in a direction from amachining start portion to a machining end portion through a gas nozzle19 moving integrally with the laser beam 27.

As a result, additional deposits around a machined hole were blown awayby the assist gas, and adhered to only a portion to be subsequentlymachined. The additional deposits were removed by the laser beam 27 at atime of the machining, and a small amount of additional deposits werefinally left at the machining end portion. The additional deposits wereremoved by a method identical with the laser beam machining method forthe wiring board described in the embodiment 6.

After ultrasonic cleaning and desmearing of the printed board 1Dobtained by the above steps, the printed board was plated with copper,and a pattern was formed thereon, thereafter observing a section. It waspossible to provide a good slit which was completely conductive throughthe plating without the residual additional deposit around the machinedhole.

As set forth above, according to the embodiment 8, a gas flow is sprayedonto the currently machined printed board 1D in the direction from thebeam irradiation start portion to the beam irradiation end portion inthe machined portion, thereby blowing away the removed material to anarea to be subsequently irradiated with the laser beam 27, anddepositing the material on a surface thereof. Since the deposit can beremoved concurrently with removal of the base material, it is possibleto reduce the removed material deposited on a surface of the printedboard 1D after the machining, and reduce the printed board cleaning stepafter the machining. Further, it is possible to significantly reduce anarea having the residual additional deposit even in machining with largevolumes of removed materials.

Embodiment 9

FIG. 22 is a typical diagram illustrating a laser beam machining methodfor a wiring board according to the embodiment 9 of the presentinvention. In the drawing, the same reference numerals are used forcomponent parts identical with those in FIG. 1, and descriptions thereofare omitted. In the embodiment 9, as a printed board 1E was used athree-layer glass epoxy printed board (FR-4) with a thickness of 200 μmand both sides coated with copper foil. Further, copper foil had athickness of 18 μm in conductor layers 2, 3, and 4. The top conductorlayer 2 was provided with no copper foil removed portion formed byetching.

A description will now be given of the operation.

A laser beam 27 was emitted from a carbonic acid gas laser with pulseenergy of 400 mJ and a pulse width of 100 μs. Further, the laser beam 27was condensed on the printed board 1E through a ZnSe lens 9 at a justfocus position at which the laser beam 27 had the minimum spot diameter,thereby irradiating with one pulse. Thereafter, at intervals of a beamirradiation pausing time of 50 ms, the printed board was irradiated withten pulses of the laser beam 27 with pulse energy of 150 mJ and a pulsewidth of 100 μs. At the time, air was used as an assist gas 10 for lensprotection, and was supplied to a machined portion at a flow rate of 10l/min through a gas nozzle 19. In the first irradiated laser beam 27,pulse energy has sufficient intensity to melt and remove the topconductor layer 2. In the second and later laser beams 27, pulse energyhas insufficient intensity to melt the top conductor layer 2.

FIG. 23 is a typical diagram showing one illustrative result ofmachining for the printed board according to the embodiment 9. In thetop conductor layer 2, copper foil including a substantially completeround with a diameter of 200 μm was removed with little effect of heaton a periphery. Further, under the conductor layer 2, the machining canbe made to form a substantially straight hole reaching the lowermostcopper foil with a small amount of projection of glass cloth 29. Afterultrasonic cleaning and desmearing of the obtained machined hole, themachined hole was plated with copper, and a pattern was formed,thereafter observing a section. It was possible to provide a goodthrough-hole with the diameter of 200 μm and a smooth inner wall.

As set forth above, even when the copper foil is not previously removedin another step such as etching, the machined portion may be irradiatedwith the pulsed laser beam 27 from the carbonic acid gas laser at thejust focus position, thereby increasing the energy density. It isthereby possible to finely remove the top copper foil with little effectof heat on the periphery. Thereafter, the printed board may beirradiated a plurality of times with the laser beam 27 having smallerpulse energy at intervals of a long beam irradiation pausing time,resulting in the through-hole with no char layer. It is thereby possibleto omit the etching serving as a previous step which is essential in theprior art, and simplify manufacturing steps. Further, both of the abovebeam irradiation conditions are identical with those described in, forexample, the embodiments 1 and 2. That is, the printed board wasirradiated with the pulsed laser beam 27 for a beam irradiation timeranging from 10 to 200 μs, most suitable for machining of the glassepoxy board, and at intervals of the beam irradiation pausing time of 15ms or more. Therefore, it is possible to provide a sharp temperaturegradient of the machined portion, and provide a platable hole with asubstantially negligible amount of projection of the glass cloth. As setforth above, even in the printed board having surfaces coated with thecopper foil and containing the glass cloth, it is possible to rapidlyand accurately provide a hole by only the laser beam machining stepwithout previously removing the conductor layer such as the copper foilon the surface of the printed board through the etching, and so forth.

Embodiment 10

FIG. 24 is a typical diagram showing a laser beam machining method for awiring board according to the embodiment 10 of the present invention. Inthe drawing, the same reference numerals are used for component partsidentical with those in FIG. 1, and descriptions thereof are omitted. Inthe embodiment, as in the embodiment 9, as a printed board 1E was used athree-layer glass epoxy printed board (FR-4) with a thickness of 200 μmand both sides coated with copper foil. Further, copper foil had athickness of 18 μm in conductor layers 2, 3, and 4. The top conductorlayer 2 was provided with a finely removed portion 30 in a range smallerthan an area to be machined.

A description will now be given of the operation.

A laser beam 27 was emitted from a carbonic acid gas laser with pulseenergy of 200 mJ and a pulse width of 100 μs.

Further, the laser beam 27 was condensed on the printed board 1E througha ZnSe lens 9 at a just focus position at which the laser beam 27 hadthe minimum spot diameter, thereby irradiating with one pulse.Thereafter, at intervals of a beam irradiation pausing time of 50 ms,the printed board was irradiated with ten pulses of the laser beam 27with pulse energy of 150 mJ and a pulse width of 100 μs. As a result, asin the embodiment 9, in the top conductor layer 2, copper foil includinga substantially complete round with a diameter of 200 μm was removedwith little effect of heat on a periphery. Further, under the conductorlayer 2, the machining can be made to form a substantially straight holereaching copper foil of the lowermost conductor layer 4 with a smallamount of projection of glass cloth.

Instead of the finely removed portion 30 shown in FIG. 24, as shown inFIG. 25, surface roughening shown by reference numeral 31 may be made toa surface of the conductor layer 2. The surface roughening utilizes, forexample, chemical treatment typically used to enhance adhesiveproperties between a resin layer and a conductor layer. The surfaceroughening to the surface of the conductor layer 2 can improveabsorption of the laser beam 27 when the copper foil having a desiredshape is removed from the conductor layer 2, and enables efficient andmore stable drilling.

As set forth above, the slight copper foil of a beam irradiated portionis previously removed by the etching, and the surface roughening is madeto the surface. The treated portions trigger the absorption of the laserbeam 27 from the carbonic acid gas laser so that the top copper foil canbe removed even when the first irradiated beam has low energy density asin the embodiment 9.

Alternatively, the two methods according to the embodiment 10 mayconcurrently be used including one method in which the slight copperfoil of the beam irradiated portion is previously removed by theetching, and the other method in which the surface roughening is made tothe surface. Alternatively, either one or both of the methods may beused concurrently with the method in the embodiment 9. In either case,as in the embodiment 9, the top copper foil can be removed even when thefirst irradiated beam has low energy density.

Embodiment 11

FIG. 26 is a typical diagram showing a laser beam machining method for awiring board, and a laser beam machining apparatus for a wiring boardaccording to the embodiment 11 of the present invention. In the drawing,reference numeral 32 denotes a laser oscillator, 33 is an fθ lens tocondense a laser beam 27, 34 is beam scanner apparatus (opticalmechanisms) using a galvanometer scanner, and 35 is a scannerdrive/laser trigger apparatus (control mechanism) to output a drivecommand for the beam scanner apparatus 34 and a trigger of laseroscillation for the laser oscillator 32.

A description will now be given of the operation.

The scanner drive/laser trigger apparatus 35 outputs the trigger of thelaser oscillation for the laser oscillator 32 at a predetermined pulsefrequency, and the drive commands for the two beam scanner apparatus 34.It is thereby possible to position a spot of the laser beam 27 at highspeed at an optional drilling position on the printed board 1F havingmany drilling positions in synchronization with a pulse frequency of thelaser beam 27 radiated from the laser oscillator 32.

A machining speed per unit time becomes higher as the pulse frequencybecomes higher. However, when irradiation with a plurality of pulses isrequired for drilling at one position, successive irradiation with ahigh pulse frequency makes a char layer thicker so that a good hole cannot be provided. For example, as seen from a relationship shown in FIG.6, the char layer is made thicker by beam irradiation at intervals of abeam irradiation pausing time less than 15 ms, that is, at a frequencymore than 67 Hz.

Thus, the spot of the laser beam 27 is sequentially moved to differentdrilling positions for each pulse. After all of the many drillingpositions in the range of a scan vision are irradiated with the laserbeam 27 pulse by pulse (after the elapse of a substantial time of 15 msor more), or after the elapse of a time of 15 ms or more fromirradiation of the first drilling position with the laser beam 27, thespot is returned to the first drilling position. The spot issequentially moved once again, and the movement is repeated severaltimes. It is thereby possible to irradiate one drilling position withthe laser beam a plurality of times while ensuring a beam irradiationpausing time of 15 ms or more for each drilling position. Therefore, ifin synchronization at a frequency of 200 Hz by using, for example, thebeam scanner apparatus 34 having the galvanometer scanner shown in FIG.26, a time of 5 ms is required for each hole. Consequently, when thescan vision includes three or more drilling positions, and the spot issequentially moved toward the positions, the beam irradiation pausingtime of 15 ms or more can be ensured for each drilling position.

As set forth above, according to the embodiment 11, even if the laserbeam 27 having a high pulse frequency is used, the beam irradiation canbe made while ensuring the beam irradiation pausing time of 15 ms ormore for each machined portion. It is thereby possible to provide a highquality platable hole having little char layer and no projection ofglass cloth. Further, a scanning frequency of the spot of the laser beam27 can be increased to its limitation so as to enable high-speeddrilling, and drill for many holes in a short time. Therefore, it ispossible to considerably improve productivity of the printed boardcontaining the glass cloth.

Embodiment 12

FIG. 27 is a typical diagram showing a laser beam machining method for awiring board, and a laser beam machining apparatus for a wiring boardaccording to the embodiment 12 of the present invention. In the drawing,reference numeral 36 means a reflection mirror disposed across anoptical axis of a laser beam 27, and 37 is an XY table on which threeprinted boards 1F are mounted, for moving the printed boards in ahorizontal plane. That is, the XY table 37 includes three machiningstations. In addition, reference numeral 38 means a control unit for theXY table 37, 39 is rotary choppers, 40 is a trigger generatingapparatus, 41 is a trigger counting portion, and ST1 to ST3 are onepulses of the laser beam 27. The laser beam machining method for thewiring board according to the embodiment 12 is used for concurrentmachining of the plurality of printed boards 1F. The embodiment will bedescribed by way of a method for the concurrent machining of the threeprinted boards 1F as one example. In the embodiment, an opticalmechanism includes the rotary choppers 39 and the reflection mirror 38,and a synchronization control mechanism includes the trigger generatingapparatus 40 and the trigger counting portion 41.

A description will now be given of the operation.

As shown in FIG. 28, in the rotary chopper 39, a disk mountedperpendicular to a rotation axis is equally divided into (3×n) areas(n=1, 2, 3, . . . ), and the equally divided areas are repeatedlymounted in the order of a reflection surface 39 a, a passing portion 39b, and a passing portion 39 b in a direction of rotation. In FIG. 28,the rotary chopper 39 is a cross-shaped reflector obtained by equallydividing a disk into (3×4) areas to have the four reflection surfaces 39a.

As shown in FIG. 27, the two rotary choppers 39 are mounted between thelaser oscillator 32 and the reflection mirror 36, and are set to rotate,with a deviation by one of the equally divided areas, in synchronizationwith each other at the same speed. Any one of the rotary choppers 39 isprovided with the trigger generating apparatus 40 to output the triggerto the trigger counting portion 41 each time the (3×n) equally dividedareas respectively move across the optical axis of the laser beam 27.The trigger generating apparatus 40 sends the generating trigger to thetrigger counting portion 41. The trigger counting portion 41 counts thereceived trigger, and sends the trigger to the laser oscillator 32 if acount value is valid (a predetermined count value is not reached). Whenthe laser oscillator 32 receives the trigger from the trigger generatingapparatus 40 through the trigger counting portion 41, the laseroscillator 32 immediately outputs the laser beam 27 by only one pulsewith a pulse width of 200 μs or less. The laser beams 27 includingoptional successive three pulses are outputted in such a manner, and aresequentially reflected at any one of the two rotary choppers 39 and thereflection mirror 36 so as to be introduced to the three machiningstations. Through a ZnSe lens 9, the three printed boards 1F arerespectively irradiated with the laser beams 27. When the predeterminednumber of triggers is counted, the trigger counting portion 41 disablesthe next and later triggers to the laser oscillator 32, and sends atable moving trigger to the control unit 38 of the XY table 37. When theXY table 37 is completely positioned, the trigger counting portion 41enables a trigger again by receiving a positioning completion signalfrom the control unit 38 of the XY table 37.

FIG. 29 is a time chart of the trigger and laser pulses in theembodiment 12. As shown in FIG. 29, the machining stations arerespectively irradiated with the laser beam 27 shown by any one ofreference numerals ST1, ST2, and ST3 once every generation of any one ofthree triggers from the trigger generating apparatus 40. For example,when the two rotary choppers 39 are rotated such that the trigger fromthe trigger generating apparatus 40 can have a cycle of 5 ms or more,the machining stations are respectively irradiated with pulses at timeintervals of 15 ms or more. As seen from a relationship shown in FIG. 6,it is thereby possible to drill for a good hole with a small amount of achar layer. When the beam irradiation is required m times for eachdrilling and sequential drilling is made for other holes, thepredetermined number of triggers in the trigger counting portion 41 maybe set to (3×m). It is thereby possible to machine the entire areas ofthe three printed boards 1F by repeating the beam irradiation andmovement of the table.

As set forth above, according to the embodiment 12, there is no decreasein energy of the laser beam 27 in the respective machining stations.Further, the speed of rotation of the rotary choppers 39 is set suchthat the laser beams 27 can be transferred to the machining stations attime intervals of 15 ms or more. It is thereby possible to concurrentlyprovide in the plurality of printed boards 1F high quality platableholes without projection of glass cloth, and more rapidly machine theprinted board 1F containing the glass cloth, resulting in a significantimprovement of productivity. Further, the combination of the beamscanner apparatus 34 in the embodiment 11 and the embodiment 12 canreduce a time required for the movement of the table, and enables higherspeed machining of the plurality of printed boards.

Embodiment 13

FIG. 30 is a perspective view showing a structure of a carbonic acid gaslaser oscillator for machining a wiring board according to theembodiment 13 of the present invention. In the drawing, referencenumeral 42 means a pair of discharge electrodes to form a dischargespace 43 in a gap therebetween, 44 is a resonator mirror, 45 is a gasflow serving as laser medium, 46 is an optical axis of a laser beam 27,and 47 is an aperture to select the degree of mode of the laser beam 27.The unit including the optical axis 46 of the laser beam 27, the gasflow 45, and a direction of discharge in an orthogonal relationship istypically referred to as three-axes orthogonal type laser oscillator.

A description will now be given of the operation.

Discharge power is fed from the discharge electrodes 42 to form thedischarge space 43 into which the gas flow 45 flows. A molecule in thegas flow 45 is excited by energy of discharge to have a gain to light.When the discharge space 43 is formed in a stationary manner, there isformed a stationary gain distribution having a peak in the vicinity ofthe downstream of the discharge space 43 as shown in FIG. 31( a).Therefore, in order to efficiently provide stationary laser oscillation,that is, successive output (CW output), it is necessary to dispose theoptical axis 46 and the aperture 47 across a line which verticallytravels in the downstream of the discharge space 43 providing themaximum gain distribution as shown in FIG. 31( b). For the purpose, inthe prior art, a typical three-axes orthogonal type of carbonic acid gaslaser oscillator has the above structure.

However, unlike the prior art, there is provided the carbonic acid gaslaser oscillator for machining the printed board according to theembodiment 13 of the present invention having a structure as shown inFIG. 32. In the structure, the aperture 47 is disposed in the range inthat the aperture 47 does not extend off the discharge space 43, and theoptical axis 46 is disposed on a line vertically travelling on thefarthest upstream side of the discharge space 43.

In a conventional structure shown in FIG. 31( b), it can be consideredthat energy of an excited molecule at a point A in the upstream of thedischarge space 43 is converted into the laser beam 27 at a time ofreaching a point B. Further, it can be seen that an excited molecule atthe point A at the moment at which the discharge is stopped is convertedinto the laser beam 27 after the elapse of a time of (X/V) (where V is agas flow rate, and X is a distance from the point A to the point B).Therefore, when the optical axis 46 is disposed in the downstream of thedischarge space 43 as shown in FIG. 31( b), a time required fordisappearance of the laser beam 27 after the discharge is stoppedbecomes longer, and a fall of a laser pulse at a time of pulseoscillation becomes slower than would be in case where the optical axis46 is disposed in the upstream of the discharge space 43 as in theembodiment 13 shown in FIG. 30. For example, in the conventional laseroscillator with a 30 mm distance between the point A and the point B,that is, a 30 mm width of the discharge electrode 42 and a gas flow rateof 80 m/s, a fall time of the laser pulse becomes 375 μs. Even if a falltime of the discharge power itself is reduced, it is impossible toreduce the fall time of the laser pulse.

On the other hand, according to the embodiment 13 shown in FIG. 32, theaperture 47 is disposed in the range that the aperture 47 does notextend off the discharge space 43, and the optical axis 46 is disposedon the line vertically travelling on the farthest upstream of thedischarge space 43. For example, for a 6.5 mm distance between the pointA and the point B and the gas flow of 80 m/s, it is possible to providean 81 μs fall time of the laser pulse. In this case, it is necessary toprovide a sufficiently short fall time of the discharge power because afall time of the laser pulse is affected by a longer fall time of thedischarge power than the fall time of the pulse. When the optical axis46 is disposed as in the embodiment shown in FIG. 32, the fall time ofthe discharge power is preferably set to 50 μs or less. When the opticalaxis 46 is disposed as in the embodiment 13 shown in FIG. 32, a risetime of the discharge power has an effect on a rise time of the laserpulse. Thus, the rise time of the discharge power is preferably set to50 μs or less in order to obtain a narrow pulse width of 200 μs or less.

As set forth above, according to the embodiment 13, it is possible toprovide the laser pulse having a sharp rise, a sharp fall, and the pulsewidth of 200 μs or less, which can not be provided by the conventionalcarbonic acid gas laser. The machining can be made without projection ofglass cloth and generation of a char layer by applying the laser pulseto the machining of the printed board.

As set forth above, according to the present invention, there isprovided the laser beam machining method for the wiring board, includingthe step of irradiating the machined portion of the wiring board withthe pulsed laser beam for the beam irradiation time ranging from 10 to200 μs and with the energy density of 20 J/cm² or more. As a result,there is an effect in that good and fine machining can be made in, forexample, the drilling for the through-hole and the blind via hole, thegrooving, and the cutting for the outside shape with respect to theboard made of the composite material having inclusion such as glasscloth.

According to the present invention, there is provided the laser beammachining method for the wiring board, including the step of irradiatingthe same machined portion of the wiring board with the pulsed laser beamwith intervals of the beam irradiation pausing time of 15 ms or more andthe energy density of 20 J/cm² or more. As a result, there are effectsin that the conduction hole can be obtained with the high aspect ratiowhich can not be obtained by the single pulse, projection of the glasscloth can be reduced, and the wiring board containing the glass clothcan rapidly and accurately be machined even in case of the multi-pulseirradiation.

According to the present invention, there is provided the laser beammachining method for the wiring board, including the step of irradiatingthe same machined portion of the wiring board with the pulsed laser beamwith the plurality of pulse groups respectively including the pluralityof pulses respectively having the energy density of 20 J/cm² or more atintervals of the pulse group interval irradiation pausing time longerthan the predetermined beam irradiation pausing time. As a result, thereare several effects of machining for the conduction hole in a shortertime than would be in the machining employing the single pulsefrequency, prevention of the temperature rise at the machined portion, areduction in gradation of the temperature gradient with respect to thedepth distance from the surface of the machined portion, and a reductionin projection of the glass cloth.

According to the present invention, there is provided the laser beammachining method for the wiring board, in which the predetermined beamirradiation pausing time is 4 ms or more, the number of pulses in thepulse group is 4 or less, and the pulse group interval beam irradiationpausing time exceeds 20 ms. As a result, there are effects of reducedgeneration of the char layer in the machined hole, and good and finemachining in the drilling for the through-hole and the blind via hole,the grooving, the cutting for the outside shape, and so forth.

According to the present invention, there is provided the laser beammachining method for the wiring board, including the step of, at a timeof scanning the surface of the wiring board while irradiating themachined portion of the wiring board with the pulsed laser beam,scanning by the laser beam such that the machined portion is notcontinuously irradiated with the laser beam over 4 pulses and atintervals of the beam irradiation pausing time less than 15 ms. As aresult, there are effects in that it is possible to reduce generation ofthe char layer in the machined hole, and increase the machining speedwhile keeping the same machining quality as that in machining bypositioning the beam at the machined portions.

According to the present invention, there is provided the laser beammachining method for the wiring board, including the steps of providingthe 1 mm beam diameter on the surface of the machined portion, andscanning the surface of the wiring board at the scanning speed rangingfrom 8 to 6 m/min while irradiating the machined portion with the laserbeam for the beam irradiation time ranging from 10 to 200 μs and atintervals of the beam irradiation pausing time of 2.5 ms. As a result,there are effects in that it is possible to increase the machining speedwhile keeping the same machining quality as that in machining bypositioning the beam at the machined portions, and provide good and finemachining such as drilling for the blind via hole in the board made ofthe composite material having inclusion such as glass.

According to the present invention, there is provided the laser beammachining method for the wiring board, including the steps of settingthe laser beam to have the square spot effective in the machining of themachined portion of the wiring board, and scanning the surface of thewiring board while irradiating the machined portion of the wiring boardwith the pulsed laser beam. As a result, there is an effect in that themachining speed can be more increased with good machining quality thanwould be in case of the circular beam.

According to the present invention, there is provided the laser beammachining method for the wiring board, including the steps of settingthe square spot of the laser beam on the machined portion to have thesize of 0.9 mm×0.9 mm, and scanning the surface of the wiring board withthe scanning speed of 6 m/min and the scanning pitch of 200 μm whileirradiating the machined portion with the laser beam for the beamirradiation time ranging from 10 to 200 μs and at intervals of the beamirradiation pausing time of 1.25 ms. As a result, there are effects inthat higher quality can be kept than would be in case of the circularbeam, and the machining speed can be increased.

According to the present invention, there is provided the laser beammachining method for the wiring board, including the steps of previouslyremoving the metallic layer on the wiring board at the portioncorresponding to the machined portion of the wiring board, forming thebase material removed portion through the machining by irradiating thebase material of the machined portion with the laser beam through themetallic layer removed portion, and additionally irradiating the basematerial removed portion and the periphery of the base material removedportion, or only the periphery of the base material removed portion withthe laser beam. As a result, there is an effect in that the rigidadditional deposit generated during the machining can simply be removedwithout the complicated step such as wet etching even in the machininghaving large volumes of removed materials.

According to the present invention, there is provided the laser beammachining method for the wiring board, including the steps of previouslyremoving the metallic layer on the wiring board at the portioncorresponding to the machined portion of the wiring board, forming thebase material removed portion through the machining by irradiating thebase material of the machined portion with the laser beam through themetallic layer removed portion, and additionally irradiating andscanning the base material removed portion and the periphery of the basematerial removed portion, or only the periphery of the base materialremoved portion with the laser beam having a smaller peak output thanthat of the above laser beam at a higher scanning speed than a scanningspeed during first laser beam irradiation. As a result, there is aneffect in that the rigid additional deposit caused during the machiningcan simply be removed without the complicated step such as wet etchingeven in the machining having large volumes of removed materials.

According to the present invention, there is provided the laser beammachining method for the wiring board, including the step of, at thetime of previously removing the metallic layer on the wiring board atthe portion corresponding to the machined portion, partially removingthe metallic layer such that the laser beam can reach only the outerperiphery of the base material removed portion to be formed byirradiating the base material of the machined portion with the laserbeam. As a result, there is an effect of good machining causing nofailure such as peeling of the metallic layer even in case of machiningwith the removed portion larger than the non-removed portion.

According to the present invention, there is provided the laser beammachining method for the wiring board, including the steps of partiallyremoving the metallic layer such that the laser beam can reach only theouter periphery of the base material removed portion to be formed byirradiating the base material of the machined portion with the laserbeam, and scanning the surface of the wiring board with the scanningspeed of 8 m/min and the scanning pitch of 100 μm while irradiating themachined portion with the laser beam for the beam irradiation timeranging from 10 to 200 μs and at intervals of the beam irradiationpausing time of 2.5 ms. As a result, there is an effect of goodmachining causing no failure such as peeling of the metallic layerbecause the base material removed portion is not entirely removed evenin case of machining with the base material removed portion larger thanthe non-removed portion.

According to the present invention, there is provided the laser beammachining method for the wiring board, including the steps of previouslyremoving the metallic layer on the wiring board at the portioncorresponding to the machined portion, and flowing the gas in thedirection from the laser beam scanning start point to the laser beamscanning end point in the machined portion at the time of the machiningby irradiating the base material of the machined portion with the laserbeam while scanning by the laser beam through the metallic layer removedportion. As a result, there are effects in that it is possible toeffectively eliminate the adverse effect of the residual additionaldeposit on the machining even in the machining with large volumes ofremoved materials, and significantly reduce the area having the residualadditional deposit.

According to the present invention, there is provided the laser beammachining method for the wiring board, including the steps of formingthe metallic layer having the desired shape by partially removing themetallic layer by pulse irradiation with the laser beam having thesufficient intensity to melt and remove the metallic layer on the wiringboard, and additionally irradiating the machined portion of the wiringboard through the metallic layer removed portion with the laser beamhaving the insufficient intensity to melt the metallic layer and thebeam irradiation time ranging from 10 to 200 μs, and including theplurality of pulses forming a train at intervals of the beam irradiationpausing time of 15 ms or more. As a result, there is an effect in thatrapid and accurate machining can be made simply by the laser beammachining step even in the wiring board having the surface coated withthe copper foil and including the glass cloth.

According to the present invention, there is provided the laser beammachining method for the wiring board, including the step of exposingthe machined portion by previously removing, through another machiningmethod such as etching, the metallic layer positioned at the targetposition for laser beam irradiation and in the range smaller than thearea to be machined. As a result, there are effects of improvedabsorption of the laser beam, and efficient and more stable drilling.

According to the present invention, there is provided the laser beammachining method for the wiring board, including the step of previouslymaking the surface roughening to the surface of the metallic layer onthe surface of the wiring board before the laser beam irradiation. As aresult, there are effects in that the metallic layer can be removed byimproving the absorption of the laser beam even in case of low peakoutput of the laser beam, and efficient and more stable drilling can bemade.

According to the present invention, there is provided the laser beammachining method for the wiring board, including the step of, at a timeof pulsed laser beam irradiation while sequentially positioning the spotof the laser beam at the target positions on the wiring board insynchronization with the pulse frequency of the laser beam, providingthe time interval of 15 ms or more between the two optional successivepulsed laser beams for irradiation of the respective target positionsirrespective of the pulse frequency by irradiating another targetposition with the pulsed laser beam outputted for the time intervaltherebetween. As a result, there is an effect in that the high qualityplatable hole can be provided with little formation of the char layerand no projection of the glass cloth because the beam irradiation can bemade with the beam irradiation pausing time of 15 ms or more ensured foreach machined portion even when the laser beam having high pulsefrequency is used. Further, there is another effect in that the scanningfrequency of the spot of the laser beam can be increased to itslimitation so as to enable the high-speed drilling, and drill for manyholes in a short time, thereby significantly improving productivity ofthe wiring board.

According to the present invention, there is provided the laser beammachining method for the wiring board, including the steps of providingthe plurality of machining stations on which the wiring boards to bemachined are mounted, sequentially dividing the pulsed laser beamoutputted from the laser oscillator among the plurality of machiningstations for each pulse, and introducing the pulsed laser beam into theplurality of machining stations at the time intervals of 15 ms or more.As a result, there are effects in that rapid machining for theconduction hole can be made at the plurality of machining stationswithout a reduction in quality of the machined hole, and productivity ofthe wiring board can significantly be enhanced.

According to the present invention, there is provided the laser beammachining method for the wiring board, in which the carbonic acid gaslaser is used as a light source of the laser beam. With the use of thecarbonic acid gas laser having high absorption coefficient shown by theglass, there is an effect of rapid and accurate machining even in thewiring board containing the glass cloth, which is difficult to machinein other lasers.

According to the present invention, there is provided the laser beammachining method for the wiring board, in which the wiring boardcontains the glass cloth. As a result, there are effects in thatprojection of the glass cloth can be reduced, and the wiring boardcontaining the glass cloth can rapidly and accurately be machined.

According to the present invention, there is provided the laser beammachining apparatus for the wiring board, including the opticalmechanism to change a direction of the laser beam and move the laserbeam on the wiring board while sequentially positioning the spot of thelaser beam at the target positions on the wiring board, and the controlmechanism for synchronous control between the pulse oscillatingoperation of the laser beam oscillator and the operation of the opticalmechanism, and control of the optical mechanism such that the timeinterval can be set to 15 ms or more between the two optional successivepulsed laser beams for irradiation of the target positions irrespectiveof the pulse frequency of the laser oscillator. As a result, there is aneffect in that the high quality platable hole can be provided withlittle formation of the char layer and no projection of the glass clothbecause the beam irradiation can be made with the beam irradiationpausing time of 15 ms or more ensured for each machined portion evenwhen the laser beam having high pulse frequency is used. Further, thereis another effect in that the scanning frequency of the spot of thelaser beam can be increased to its limitation so as to enable thehigh-speed drilling, and drill for many holes in a short time, therebysignificantly improving productivity of the wiring board.

According to the present invention, there is provided the laser beammachining apparatus for the wiring board, including the opticalmechanism to sequentially divide the pulsed laser beam outputted fromthe laser oscillator among the plurality of machining stations for eachpulse and introduce the pulsed laser beam into the plurality ofmachining stations for each pulse at time intervals of 15 ms or more,and the synchronization control mechanism for synchronous controlbetween the dividing operation of the optical mechanism and the pulseoscillating operation of the laser oscillator. As a result, there is aneffect in that rapid machining for the conduction hole can be made atthe plurality of machining stations without a reduction in quality ofthe machined hole.

According to the present invention, there is provided the laser beammachining apparatus for the wiring board, including the opticalmechanism provided with the at least one rotary chopper rotated at thepredetermined speed of rotation, having the plurality of reflectionsurfaces and the plurality of passing portions at positions equallydividing the periphery about the axis in the plane perpendicular to therotation axis, and the synchronization control mechanism provided withthe trigger generating apparatus to generate the trigger each time allthe equally divided areas including the plurality of reflection surfacesand the plurality of passing portions in the rotary chopper respectivelymove across the optical axis of the laser beam. As a result, there is aneffect in that rapid machining for the conduction hole can be made atthe plurality of machining stations without a reduction in quality ofthe machined hole.

According to the present invention, there is provided the carbonic acidgas laser oscillator for machining the wiring board, in which the lengthof the discharge space in the gas flow direction is equal to or morethan the width of the aperture, the optical axis passing through thecenter of the aperture is set to be positioned in the range that theentire area of the aperture does not extend off the area extending inthe gas flow direction of the discharge space and on the farthestupstream side of the gas flow, and the rise time and the fall time areset to 50 μs or less in the discharge power fed to the discharge space.As a result, there are effects in that it is possible to reduce the riseand the fall of the laser beam, and obtain the laser beam with the shortbeam irradiation time suitable for machining of the wiring board.

While preferred embodiments of the invention have been described usingspecific terms, such description is for illustrative purposes only, andit is to be understood that changes and variations may be made withoutdeparting from the spirit or scope of the following claims.

1. A laser beam machining method for a wiring board, using a laser beamfor machining such as drilling for a through-hole and a blind via hole,grooving, and cutting for an outside shape of the wiring board with ametallic layer formed on a base material surface, and the methodcomprising the steps of: forming the metallic layer having a desiredshape by partially removing the metallic layer by pulse irradiation witha laser beam having sufficient intensity to melt and remove the metalliclayer; and additionally irradiating a machined portion of the wiringboard through the metallic layer removed portion with the laser beamhaving insufficient intensity to melt the metallic layer and a beamirradiation time ranging from 10 to 200 μs, and including a plurality ofpulses forming a train at intervals of a beam irradiation pausing timeof 15 ms or more.
 2. A laser beam machining method for a wiring boardaccording to claim 1, wherein the machined portion is exposed bypreviously removing, through another machining method such as etching,the metallic layer positioned at a target position for laser beamirradiation and in the range smaller than an area to be machined.
 3. Alaser beam machining method for a wiring board according to claim 1,wherein surface roughening is previously made to a surface of themetallic layer on a surface of the wiring board before the laser beamirradiation.